1. Field of the Invention
The present invention relates to a bias circuit for a power amplifier and, more particularly, to a bias circuit including an emitter follower and capable of operating by a low external reference voltage a power amplifier formed on a GaAs chip integrally with the bias circuit.
2. Background Art
In recent years, a GaAs-HBT power amplifier (HBT: heterojunction bipolar transistor) is being widely used as a power amplifier for portable telephones in code division multiple access (CDMA) systems or other systems, or as a wireless LAN power amplifier.
For example, in Japanese Unexamined Patent Publication No. 2004-343244, a circuit formed as a conventional GaAs-HBT power amplifier module is disclosed.
FIG. 11 shows an example of a circuit formed as a conventional GaAs-HBT power amplifier module. A portion surrounded by the broken line is formed on a GaAs chip. Other portions are formed of chip parts and lines on the module substrate.
The circuit shown in FIG. 11 has an RF signal input terminal (IN) and an RF signal output terminal (OUT). Tr1 and Tr2 respectively denote heterojunction bipolar transistors (HBTs) in initial and final stages. Vc1 and Vc2 respectively denote terminals for power supply to the collectors of the transistors Tr1 and Tr2. Vcb denotes a terminal for power supply to a bias circuit for the transistors Tr1 and Tr2. Rb1, Rb2, Rb12 and Rb22 denote resistors. C1 to C4, C21 to C23, Cd1, Cd2 and Cdb denote capacitors. L1 and L2 denote inductors. L11 and L21 to L23 denote lines having particular electrical lengths and functioning as inductors.
FIG. 12 shows a concrete example of a circuit configuration of the final stage transistor Tr2 and the bias circuit section for this transistor. (The initial transistor Tr1 and the bias circuit section for the initial transistor Tr1 also have basically the same circuit configuration.) Trb1 to Trb5 denote HBTs. Rbb1 to Rbb6 denote resistors. Vrefb denotes a terminal for input of a reference voltage to the bias circuit (a terminal through which a reference voltage is externally supplied). Vcb denotes a collector power terminal for the bias circuit. Vbo denotes an output terminal of the bias circuit. The bias circuits for the Tr1 and Tr2 operates so as to constantly maintain the idling currents through the Tr1 and Tr2 of the power amplifier (bias currents when no RF signal input is supplied) under varying temperature
For the normal operation of the bias circuit for the power amplifier shown in FIGS. 11 and 12, a reference voltage Vref higher than twice the barrier voltage of HBTs, typified that in a two-stage cascaded section formed of Trb4 and Trb5 in a diode connection or that in a two-stage cascaded section formed of the final stages Tr2 and Trb1, is required. That is, in a case where GaAs HBT is used, a Vref of about 2.7 to 2.9 V is required from consideration of a barrier voltage of about 1.25 to 1.30 V and a voltage drop across the resistor Rbb1 (about 0.2 to 0.3 V).
However, when Vref is reduced, for example, to a voltage of 2.5 V lower than twice the barrier voltage of the HBTs, an idling current cannot flow even at ordinary temperature and the bias circuit shown in FIGS. 11 and 12 is practically unusable unless it is modified. If the circuit operates at a lower temperature, the problem that an idling current cannot flow is more serious. This is because the barrier voltage determined by the device material is higher if the temperature is lower. Ordinarily, the barrier voltage has a gradient of about −1 to −2 mV/° C.
To solve the above-described problem, the inventors of the present invention have devised a circuit configured as shown in FIG. 13 and capable of operating at a low voltage: Vref=2.4 to 2.5 V. In this circuit shown in FIG. 13, Tr2a and Tr2b denote power transistors; Trb1 to Trb6, biasing transistors; Rbb1 to Rbb13, resistors; and C4 and Ca, capacitors. This circuit is characterized in that Tr2a has a line for injecting a current directly through the resistor Rbb9 (current drive) and a line for supplying a current through an emitter follower formed by Trb1 and Trb2 (voltage drive), and that Tr2b has only a line for injecting a current through the resistor Rbb13. If this circuit is used, power amplification at a low voltage: Vref=2.4 to 2.5 V can be performed.
In the current shown in FIG. 13, however, there is a possibility that, if factors including circuit constants are not sufficiently optimized, the gain (Gp) and distortion characteristics (ACP/ACLR) will degrade at a low temperature, as shown in FIG. 14, although there are no gain distortion problems at ordinary or high temperature. A reduction in gain Gp under such conditions has been experimentally observed in correspondence with degradations in distortion characteristics.